Visual quality is an important aspect of the user experience in many media applications. Furthermore, in compression/decompression (codec) systems, visual quality may be primarily based on the compression format used. For example, a video encoder compresses video information so that more information can be sent over a given bandwidth or stored in a given memory space or the like. The compressed signal or data may then be decoded via a decoder that decodes or decompresses the signal or data for display to a user. In some implementations, constant visual quality may be desirable. Constant visual quality, as its name implies, aims to achieve a fixed subjective quality across video frames or sequences of video frames. Typically, constant visual quality implementations may employ a variable bitrate such that file sizes or the sizes of various portions of a bitstream may be variable depending on factors such as the complexity of the video being compressed.
In one approach, a constant quantization parameter (QP) is used to achieve constant visual quality. For example, transform coefficients obtained via an encoding technique such as the H.264/MPEG-4 Advanced Video Coding (AVC) standard or the High Efficiency Video Coding (HEVC) standard or the like may be quantized using a constant QP. For example, larger QP values provide greater compression at the cost of lesser quality while lower QP values achieve greater visual quality at the cost of lesser compression. However, such constant QP approaches may not fully provide constant visual quality as perceived by users. Furthermore, such constant QP approaches may be wasteful in compressing certain portions of video sequences (either spatially within video frames or temporally across video frames).
As such, existing techniques may not provide constant visual quality for users and/or effective compression. Such problems may become critical as the transmission of video at ever higher quality becomes more widespread.